Vajiralongkorn, 64, accepted the throne of politically troubled nation following an invitation from the head of the National Legislative Assembly (NLA), becoming the 10th monarch of the Chakri dynasty that dates back to 1782.
"I agree to accept the wishes of the late king... For the benefit of the entire Thai people," he said tonight, two days after he was proclaimed King Rama X.
The event was broadcast on all Thai television channels.
Buddhist temples across the country have been instructed to beat drums and gongs after his proclamation.
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He succeeds his father King Bhumibol Adulyadej, also called King Rama the IX who died in October aged 88 and was the world's longest-reigning monarch.
Vajiralongkorn's accession to the throne ends a period of uncertainty during which Privy Council President Gen Prem Tinsulanonda, a 96-year-old former prime minister, was serving as regent.
He ascends the throne 50 days after King Bhumibol Adulyadej's death.
Many school representatives from the provinces headed to Bangkok yesterday to buy portraits of the Crown Prince as well as flags with the Crown Prince's emblem. They are preparing decorations to celebrate the new King's succession.
Besides photographs and flags, clothes with logos and messages to welcome the new King are also being stocked to meet the expected high demand.
Earlier, Prime Minister Prayut led members of the Cabinet and government officials in front of Government House to mark the 50th day of mourning for King Bhumibol.
The ceremony began with the abbot of
Ratchabopitsathitmahasimaram temple, leading 89 monks in chanting prayers.
Gen Prayut then led cabinet members and government officials to give alms to the monks.
A similar ceremony was held in front of the the office of the Bangkok Metropolitan Administration, where governer Aswin Kwanmuang led city officials and members of the city and district councils by giving alms to monks.